The librsync library itself can be found at http://librsync.sourceforge.net/ and implements rsync remote-delta algorithm (http://rsync.samba.org/tech_report).

This algorithm is using a rolling check-sum and a md4 check-sum for
blocks of the file to generate a signature file. The signature file is used
to generate a diff from the file on the remote site. This diff can be
used on the local site to patch the file.
The Algorithm is from rsync 1.x and not compatible with rsync 2.x.